Various methods of making perforated polymer films are known. For example, U.S. Pat. No. 3,012,918 (Schaar), and British Patent Specification Nos. GB 851,053 and GB 854,473 all generally describe processes and apparatuses for improving the heat-sealability of polymeric films by passing the film over a cooled, hollow, rotating, metal cylinder or support roll with a desired perforation pattern while a jet of gas-heated air is directed onto the surface of the film so that specific areas of the film are melted, forming a pattern of perforations. The preferred linear speed of the film/web during the process is between 4-33 yards per minute. The apparatus in Schaar also includes a cooling jet of air directed at the cylinder surface, operating to maintain the surface temperature of the cylinder between 55 to 70° C.
U.S. Pat. No. 3,394,211 (MacDuff) discusses flame perforation of heat-shrinkable, biaxially oriented polypropylene films using a method and apparatus similar to U.S. Pat. No. 3,012,918 (Schaar) with the improvement of restraining the edges of the film by either adhesive or frictional engagement means, thus preventing transverse and/or longitudinal shrinkage during the perforation process. MacDuff also utilizes a heated air exhaust vent and a stream of cooling air to cool the surface of the support roll. The restraining system combined with the exhaust and cooling air system eliminate the need for a complex cooling system for the support roll/cylinder.
British Patent Specification No. GB 1,012,963 discloses a method and apparatus for flame perforating any suitable thermoplastic film capable of being softened and melted by heat. In GB 1,012,963 the tip of the flame just impinges on the outer surface of the plastic film as the film is slightly stretched and passes over a liquid coolant-chilled rotating cylinder, while the film is moving at a linear speed of approximately 10 yards per minute. The rotating cylinder has a pattern of indentations, which together with the flame promote the perforation of the film via the low heat conductivity of the air trapped behind the film in the indentations of the cylinder. The flame and burner in GB 1,012,963 are positioned at about mid-point of the segment of contact between the film with the cylinder surface.
British Patent Specification No. GB 1,083,847 teaches a method and apparatus for creating a net-like structure of polymer film by first forming protrusions in the film using heated pins on a nip roller, then biaxially stretching the film, flame perforating the protruding portions of the film as it passes over a chilled cylinder, using a process similar to GB 1,012,963 and finally biaxially stretching the film a second time.
Additionally, technical literature reports that flame treatment effectiveness increases as the flame-to-film distance decreases until the tip of the luminous cone of the flame reaches the poly(olefin) film surface, see for example Flame Surface Modification of Polypropylene Film, Strobel et. al., J. Adhesion Sci. Technology, Vol. 10, No. 6, page 529 (1996)
U.S. Pat. No. 5,891,967 (Strobel et. al.) discusses a flame-treating method of modifying a polymeric substrate, where the optimal distance of the flame to the film surface is generally less than 30 mm and can be as low as −2 mm, meaning approximately 2 mm of the tip of the luminous flame actually impinges the film surface. However, U.S. Pat. No. 5,891,967 also discloses that the distance is preferably between 0 mm and 10 mm and more preferably between 0 mm and 2 mm.